I think its the damper plate
Hello, Thanks for the note. My guess is that you have a damper plate, which is being compressed to its fullest amounts during the reduction of Rpm’s. The damper plate is the connection device, which is attached to the flywheel and accepts the transmission splined shaft. The purpose of the damper plate is two fold. First is to reduce the start up moment to the transmission gearing. Without the flexible damper plate your flywheel is rotating, you place the trans into gear, boom the gears have an instant impact and teeth are sheared off. With the damper in place it’s a gradual start up to the teeth (10 mille seconds verses 1 mille second) helping to remove the shock load to the crankshaft and transmission teeth. The second important function of the damper plate is to help reduce gear clatter/rattle. Because the flywheels on modern engines are less in weight, they do not produce much inertia to help in constantly rotating the transmission while it’s in gear. As you begin to reduce the Rpm’s to the engine the flywheel slows down but the propeller and it’s pitch, running gear, and speed of the vessel overcome the flywheels speed of rotation causing the damper to completely compress, completely decompress and so on and so on. The compress and decompress of the damper springs is a major source of vibration and noise in a vessel, which is almost always heard at the slow Rpm range of the motor and not at the higher rotation speeds. When placing adverse loads on the running gear like crash turns or following seas, which pick up the stern accelerate the vessel then decelerate the boat as it passes, can also cause major trans noise. The possible solution is to replace the present damper with a unit with a higher amount of deflection. (the amount of compression in a degrees before the transmission rotation tries to advance the engine) Your current damper, if brand new, may only have 3 degrees of deflection, where as a hi deflection unit may have 33 degrees. The older the damper gets the less deflection it will have. If it was my boat I would check that the shaft is not making contact to the stern tube and then let a transmission person hear the noise, which has experience with dampers and the noise they make. What you have described is very common problem in the workboat industry, which is often fixed by replacing the damper. They often run their diesels at very slow Rpm’s getting a ton of gear lash noise. Please feel free to call me if you do not understand what I have written. Of coarse all of this is my opinion and one of many. Hope this helps. Kevin
